Use react-hook-form and zod in add modals
Add @hookform/resolvers and zod-based validation to
AddFile/AddSecret/AddVariable/AddVolume
modals (useForm, zodResolver, register, handleSubmit, reset, show
errors). Refactor
useAddFile/useAddSecret/useAddVariable/useAddVolume hooks to accept a
payload object
(sandboxId, name/path, value/content) for mutationFn. Implement
DeleteSandboxModal
delete handler and update confirmation copy. Update dependencies (add
@hookform/resolvers).